Energy beams, such as laser beams, are commonly used in numerous different types of machining operations, including what are often referred to as micro-machining operations. In conventional micro-machining operations, laser beams are used to machine features on substrates, such as printed circuit boards and panels. One typical micro-machining operation involves the laser trimming of resistors and capacitors printed on a substrate.
FIG. 1 depicts a conventional laser beam micro-machining system 10 of the type used for resistor trimming. The system 10 includes a laser beam emitter 15 operating under the control of the system controller 55. The emitted laser beam 20 passes through a fixed beam expander 25 to the scan subsystem 30.
As shown, the scan subsystem 30 includes beam directing optics 35 and a focus lens 40. The scan system 30 also operates under the control of the system controller 55. The beam 20 is directed by the beam directing optics 35 through the focus lens 40. The beam directing optics 35 includes a pair of galvo driven mirrors. The focus lens 40 has a field of view and focuses the beam 20 so that the beam 20 impinges point 22 on a resistor 45 on the substrate 50 to perform the trimming operation. An X-Y stage 52 is provided to move the substrate 50 in two axes to position the resistor under the field of view of the lens 40. The focal point 22 of the directed beam impinges on the substrate 50, so as to perform the trimming operation.
The fixed beam expander 25 establishes the size (effective entrance pupil) of the beam entering the scan system 30. As shown in FIG. 1, using the fixed beam expander 25 results in the collimated beam entering the scan subsystem having a pupil size B which corresponds to a spot size B′ of the beam at the focal point 22 on the focal surface. Thus, should a larger or smaller spot size be desired, the fixed beam expander 25 must be replaced with another beam expander having the appropriate beam expansion ratio to achieve the desired spot size.
Generally, if a larger spot size is desired at focal point 22, a fixed beam expander that will cause the effective entrance pupil size of the beam entering the scan subsystem to be less than B must be substituted for the fixed beam expander 25. On the other hand, if a smaller spot size is desired at the focal point 22, a fixed beam expander that will cause the effective entrance pupil size of the beam entering the scan subsystem to be greater than B must be substituted for the fixed beam expander 25. In any such cases, the beam expander must be changed.
In addition, it has recently become necessary for such machines as shown in FIG. 1 to be used in machining features on substrates having various sizes and/or various shapes. Varying the size and/or shape of the substrate will result in a need to vary the size of the scan field that must be covered by a focused beam having the desired spot size. The step size of the X-Y stage movement may also have to correspondingly change. Accuracy of beam placement and the degree of telecentricity may also vary with the changes in the field size.
The achievable spot size within the scan field in a conventional laser machining system is governed by a number of factors, including the pupil size and the focus lens characteristics. Relevant focus lens characteristics include the lens focal length, the lens performance degradation at large angles, the telecentricity of the focus lens, and the complexity and cost of the focus lens.
With regard to degradation, it will be understood that focusing a beam through the outside area of the focus lens will often result in degraded performance when smaller spots are desired. In addition, beam positional accuracy may be degraded near the perimeter of the focal surface when the field size is increased. Moreover, in a non-telecentric system, telecentricity may also be degraded near the perimeter of the focal surface at increased field sizes.
To address a large scan field, a focus lens having a large field of view and a long focal length must be used. Without increasing the beam pupil size, this will result in the beam having a large spot size at the focal surface. However, if the pupil size is increased, the size of the mirrors or other beam directing components must also be increased, thereby degrading dynamic performance. To achieve a smaller beam spot size at the focal surface, without increasing the focal length, the beam pupil size at the beam directing optics must be increased.
For some scan field applications, a focus lens having a high degree of telecentricity may be required. However, if the packing density of the features to be machined is high, resulting in the need for a small beam spot size at the focal surface, then the complexity and cost of designing and manufacturing such a focus lens can be quite high.
Accordingly, conventional approaches to machining features at varying packing densities on substrates having different sizes and/or shapes is to utilize separate systems, with a fixed field size and X-Y stage step size, optimized for each particular machining application, as required. A separate focus lens for different fields is required. The different focus lenses are manually installed and de-installed in the scan subassembly prior to initiating of the machining of features for a particular job. This results in increased manufacturing costs.
Hence, the need exists for a machining system capable of machining features at various packing densities on substrates having different sizes and/or shapes without resorting to a different system or manually changing the focus lens in the scanning subassembly.